1. Field of the Invention
The invention relates to an audio system comprising a circuit for processing an audio signal, whereby the circuit comprises an input for receiving the audio signal and an output for supplying an output signal, a harmonics generator coupled to the input for generating harmonics of the audio signal, and adding means coupled to the input as well as to the harmonics generator for supplying a sum of the audio signal and the generated harmonics to the output.
The invention further relates to a circuit for processing an audio signal, a harmonics generator and a method for processing an audio signal.
2. Description of the Related Art
An audio system according to the preamble is known from European Patent Application EP-A 546 619. Since the invention of the electrodynamic loudspeaker, there has been a need for greater acoustical output, especially at low frequencies. Often, however, for instance, in television sets or portable audio sets, this acoustical output is severely limited by the small size of the loudspeakers. It is known that this dilemma can be solved by using a psychoacoustic phenomenon often referred to as virtual pitch or missing fundamental, which evokes the illusion of a higher bass-response, while the loudspeaker does not radiate more power at those low frequencies. This illusion can be created by replacing low-frequency tones, which are present in the audio signal but cannot be reproduced by a small loudspeaker, by harmonics of these tones. The harmonics now represent the low-frequency tones.
In the known audio system, a low-frequency band of an audio signal is selected and supplied to a harmonics generator for generating harmonics of the selected signal. The generated harmonics are thereafter added to the audio signal. In this way, the low-frequency perception of the audio signal is improved. In the known audio system, a full-wave rectifier is used as the harmonics generator, which generates only even harmonics. A drawback of the full-wave rectifier is that the amplitude of the generated harmonics decreases rapidly with the number of the harmonic, e.g., with respect to the second harmonic, the amplitudes of the fourth, sixth and eighth harmonics are, respectively, 14 dB, 21 dB and 26 dB lower. Because of this reduction in amplitude of the generated harmonics, the virtual pitch effect cannot be fully exploited in the known audio system.
An object of the invention is to provide an audio system, wherein the harmonics generator is capable of generating harmonics, the amplitudes of which are substantially equal to each other. This object is achieved in the audio system according to the invention, which is characterized in that the harmonics generator comprises an integrator for integrating the audio signal, and, coupled thereto, resetting means for resetting the integrator at resetting times.
By integrating the audio signal and resetting the integrated signal at resetting times, a non-symmetrical waveform is obtained which comprises both odd and even harmonics, whereby the amplitude of the generated harmonics decreases relatively slowly with the number of the harmonic. Consequently, in the audio system according to the invention, there is a relatively strong virtual pitch effect. Furthermore, because the amplitude of the generated harmonics is proportional to the amplitude of the audio signal, no annoying distortions in the output signal are introduced by the harmonics generator.
An embodiment of the audio system according to the invention is characterized in that the resetting means is embodied so as to periodically reset the integrator according to a reset period. By virtue of this measure, the generation of harmonics is repeated periodically, thus providing a constant stream of harmonics in the output signal.
A further embodiment of the audio system according to the invention is characterized in that the resetting means is embodied so as to determine the reset period in dependence on the period of the audio signal. This is a simple embodiment of the audio system according to the invention.
A further embodiment of the audio system according to the invention is characterized in that the resetting means is embodied so as to reset the integrator during at least a part of the reset period. By virtue of this measure, it is possible to prevent certain parts of the audio signal, for example, those parts where the amplitude of the audio signal is negative, from being integrated.
A further embodiment of the audio system according to the invention is characterized in that the resetting means is embodied so as to reset the integrator when the audio signal crosses a threshold value. By virtue thereof, integration of those parts of the audio signal which exceed a certain threshold value can be prevented.
A further embodiment of the audio system according to the invention is characterized in that the harmonics generator further comprises a rectifier for rectifying the audio signal, whereby the rectifier is coupled to the integrator so that the rectified signal is integrated by the integrator. By virtue of this measure, also the negative parts of the audio signal contribute to the amplitude of the generated harmonics.
Some low-frequency tones, which are reproduced by the audio system according to the invention, are perceived by human beings as having a higher loudness than the loudness of the corresponding low-frequency tones which are present in the audio signal. In order to compensate for this undesired artefact, a further embodiment of the audio system according to the invention is characterized in that the integrator is embodied so as to limit the amplitude of the integrated signal. In this way, the perceived loudness of low-frequency tones can be controlled, preferably, in such a manner that the perceived loudness is substantially equal to the original loudness.
A further embodiment of the audio system according to the invention is characterized in that the integrator is embodied so as to stop the integration in dependence on the amplitude of the integrated signal. This is a simple and effective embodiment for limiting the amplitude of the integrated signal and thus the perceived loudness of low-frequency tones.
A further embodiment of the audio system according to the invention is characterized in that the integrator is embodied so as to adapt an integration time-constant in dependence on the amplitude or the frequency of the integrated signal. By virtue of this measure, the amplitude of the integrated signal can be limited gradually, enabling a smooth control of the perceived loudness of low-frequency tones.